From: Allison Winter, ENN
Published January 10, 2013 09:57 AM

Study suggests magma forms deeper than previously thought

A group led by Rajdeep Dasgupta, geologist and assistant professor of Earth science at Rice University, put samples of peridotite, a dense igneous rock, under pressure in a Rice University laboratory and found that rock can and will liquefy, as deep as 250 kilometers in the mantle beneath the ocean floor. These recent findings provide new evidence that magma can form at a depth much deeper than scientists once thought.

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The mantle is a highly viscous layer between the Earth’s crust and it’s outer core. In this rocky buffer, magma rises with the convective currents, cools, and spread out to form the ocean crust. The Rice team focused on the mantle beneath the ocean because that's where the crust is created and subsequently where “the connection between the interior and surface world is established," says Dasgupta.

From measuring the speed of seismic waves after an earthquake, scientists can determine the mantle's density. "Seismologists have observed anomalies in their velocity data as deep as 200 kilometers beneath the ocean floor," Dasgupta said. "Based on our work, we show that trace amounts of magma are generated at this depth, which would potentially explain that." Scientists previously thought that melting started occuring at 70 kilometers beneath the seafloor.

A previous study by Dasgupta determined that the presence of carbon dioxide causes melting in the Earth's deep upper mantle. The present study shows that carbon not only leads the charge to make carbonate fluid but also helps to make silicate magma at significant depths.

"Scientists knew the effect of a trace amount of carbon dioxide or water would be to lower this boundary, but our new estimation made it 150-180 kilometers deeper from the known depth of 70 kilometers," he said.

"What we are now saying is that with just a trace of carbon dioxide in the mantle, melting can begin as deep as around 200 kilometers. And when we incorporate the effect of trace water, the magma generation depth becomes at least 250 kilometers. This does not generate a large amount, but we show the extent of magma generation is larger than previously thought and, as a consequence, it has the capacity to affect geophysical and geochemical properties of the planet as a whole."

Experimental petrology allows scientists to simulate processes that can take millions of years in a matter of minutes thus explaining and predicting past, present, and future geophysical processes.

Read more at Rice University.

Earth image via Shutterstock.

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